RuBisCO, D-ribulose-l,5-bisphosphate carboxylase-oxygenase (EC 188.8.131.52), converts carbon dioxide to sugar in the first step of photosynthesis 1,2. In plants and some bacteria, this enzyme has an L 8S8 structure, where L is the large catalytic subunit and S is the small subunit of unknown function. The molecule resembles a keg 105 Å along the 4-fold axis and 132 Å in diameter at the widest point of the keg3-6. Here we describe the quaternary structure of RuBisCO from N. tabacum, the first L8S8 type known from an X-ray crystallographic study at near-atomic resolution (3 Å). The structure shows that all eight L subunits are elongated along the 4-fold axis so that the molecule cannot be simply described as layers of subunits, as it had been from studies by electron microscopy5,6. The structure, with its elongated and inter-digitated L subunits, is evidence against a large, sliding-layer conformational change in plant RuBisCO, as proposed recently in Nature for the same enzyme from Alcaligenes eutrophus7.
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